PARIS — The French military intervention in Mali has increased the threat of domestic terrorism, with some French citizens of black African ancestry becoming more willing to fight under the banner of jihad, according to France’s most prominent investigative judge dealing with terrorism.

The concerns of the French authorities were once largely limited to residents of North African Arab ancestry, like Algerians and Tunisians, but attention has broadened to include those French with roots in Mali, Senegal, Nigeria and Niger, the judge, Marc Trévidic, said in an interview.

At the same time, since a Frenchman of Algerian ancestry, Mohamed Merah, 23, killed seven people last March in Toulouse, the French police and intelligence agencies have been opening more investigations but have not been given more investigators, and have also become less willing to monitor terrorism suspects for longer periods of time before intervening and detaining them, Mr. Trévidic said.

“After Merah,” who said he had been recruited into Al Qaeda, “the French are afraid of terrorism,” Mr. Trévidic said. “They are afraid that you can have one or two or three Mohamed Merahs. And they could be right; no one really knows.”

Mr. Trévidic, 47, has been dealing with terrorism cases since 2000, before Al Qaeda’s attacks in the United States, and he is perhaps the best-known of the eight investigating magistrates assigned to a special antiterrorism court in Paris. When he started, he said, terrorism was simpler — “there were no women, no children and few groups.”

Now, “the field of suspects is much larger, so the situation gives me a little fear,” he said. “We’re fighting groups that are less powerful and organized than before, but which are much more difficult to detect.”

The Mali intervention has raised the stakes, he said, at home and abroad, where French tourists and workers are more vulnerable to kidnapping. When France said several months ago that it would intervene in Mali, with the help of African forces, “at that moment young Muslims in France heard that Shariah is in force in northern Mali and they wanted to go there to defend this ‘real Islam’ against an announced intervention,” he said.

The authorities began to notice an increase in the number of French passport holders departing for Mali. Mr. Trévidic is now dealing with the case of four Frenchmen, at least two of whom are in Mali, he said, who are believed to be fighting alongside the militants. As the intervention continues, he said, “now the main problem is to try to stop the departures, because if we can’t, the threat will be higher and higher.”

“Because they will be trained and come back and organize themselves,” he added.

For the moment, he said, the threat is not large, because these groups are small and disorganized. “But we have a lot of citizens in France who are also Malians, Senegalese, Nigerians and Nigeriens, and they have passports and can also go there, and the frontiers are very long and fluid, so it will be very difficult.”

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Mohamed Merah, 23, French of Algerian ancestry, killed seven people last March in Toulouse. More violence like his is feared.

Just this month, the police arrested a man from Senegal, “and we know him because his brother went to Mali, and we think he went to Mali, too,” the judge said. “Of course he was arrested, and we don’t know exactly yet what he intended to do.”

About 50 or so French Muslims have traveled to Syria to fight, he said, “to join who knows what group.”

Mr. Trévidic has been criticized for his outspokenness, but he is more considered than a predecessor, Jean-Louis Bruguière. An investigating magistrate in France holds great power, working with and directing the police, gathering evidence both for and against the accused. The work is especially delicate when the issue is terrorism.

Mr. Trévidic fears that France is falling behind the threat, with a system devised to look at larger, better organized groups. The case of Mr. Merah, who had been under surveillance for some time, shocked the authorities. Few cases were opened before his attacks, and the resources of the police and the judiciary were cut to save money. After the shootings, many cases were opened but without sufficient personnel to handle them, Mr. Trévidic said.

Another consequence, he said, is that the French counterintelligence agency, the D.C.R.I., is more reluctant to conduct lengthy surveillance on suspects who have been abroad, fearing that, like Mr. Merah, they will commit a crime in the meantime. “After Merah, our policemen are afraid,” Mr. Trévidic said. “They don’t want to monitor people for a long time after they come back, because if they monitor someone and this guy commits a bomb attack it will be terrible a second time for D.C.R.I.”

But quick detentions are bad for the investigation, because there is less chance to discover a suspect’s contacts or plans. “If they arrest the guys immediately, the evidence is thin for us, and after all, we are a judicial system,” he said. Since Mr. Merah, he said, many suspects have been arrested, but at least 20 potential cases have been thrown out for lack of evidence.

The suspects are also younger, he said, and angrier. “The young Muslims I see in my office have developed a kind of paranoia,” he said. “They are sure that we want to fight Islam, that we’re against Islam. They were born in France and were not practicing Muslims, but now they pray and they are sure we are against Muslims.”

Part of the problem, he said, is the lack of religious education in public schools, which are secular and rarely provide comparative religion courses. “So no one tells them about their own story, their own origins,” he said. “They have only what they can pick up themselves on the Internet or from some friends, and where there is no foundation, you can go to extremism very quickly.”

Young second- and third-generation Muslims in France are often badly integrated, live in largely segregated, poor suburbs and feel at home nowhere. “So some find their pride in religion now, in extremism,” Mr. Trévidic said. And they are left out of the national conversation, he added. Even on issues like banning the niqab, or full facial veil, “there is no discourse for the Muslims, for those who want to wear the niqab, to explain that their religion does not require it,” he said. “For all these problems it’s always the same. We talk only to the people who agree with us.”

For now, “the danger is not so big, perhaps, but the threat is big,” he said. “This is the way terrorists win — they can win with very little.”

A version of this article appears in print on February 24, 2013, on page A11 of the New York edition with the headline: French Intervention in Mali Raises Threat of Domestic Terrorism, Judge Says. Order Reprints|Today's Paper|Subscribe